The present invention relates to an improved process for the removal of acid gases including NO.sub.x from exhaust gases, and particularly to a combined removal of NO.sub.x and SO.sub.2 from flue gas and the like. (Flue gas usually contains both NO and NO.sub.2 ; these oxides of nitrogen are collectively given as NO.sub.x.)
Concerns about air pollution caused by acid rain are increasing world wide, and considerable research effort is being expended to provide effective treatment of flue gases and other exhaust gases to remove acid forming components therefrom. However, the present methods have disadvantages which are particularly acute with respect to the removal of NO.sub.x.
Early methods were primarily used to remove pollutants when the concentrations were very high. As time goes by, and larger volumes of gases are generated, tolerable levels of emissions keep getting lower and lower. At this time emissions may be treated to obtain acceptable levels of SO.sub.2 by means of scrubbing processes using aqueous solutions. However, removal of NO.sub.x presents problems, the most serious being sufficient removal and economic considerations. In addition, the economics of using two processes has prompted efforts to utilize wet scrubbing for removal of both NO.sub.x and SO.sub.2 in a single process, and some success has been achieved in this direction. Due to the difficulty in solubilizing NO in aqueous solution, these processes have utilized expensive ingredients and often have provided other products requiring disposal.
Wet processes developed for removal of NO.sub.x have been reported. For example, Patent No. P 32 38 424.6 issued by the Federal Republic of Germany Apr. 19, 1984 to Hoechst AG utilizes red phosphorus in inert oxidizing media to remove NO and NO.sub.2 from flue gas. However, the patent reports the treatment of very high concentrations of NO, typical concentrations being up in the thousands of parts per million, and in Example 7 of the patent where 1000 parts per million were treated, only 40% was removed. In the two part Example 9, the patentee reports 14,000 parts per million were treated in the first step to obtain a 90% removal to 1,300 parts per million; and in the second part about a 65% removal to about 460 parts per million. Such effluent concentrations are not sufficiently low enough, and we have found that red phosphorus is not satisfactory to treat concentrations of 500 parts per million or less.
Standards recently set in the State of California for emissions from power plants fueled by natural gas is 20 parts per million or less for NO in northern California and 10 parts per million or less in southern California. At this time, federal standards are 75 parts per million or less. Such standards may be attainable using the selective catalytic reduction (SCR) process which is very expensive. Other approaches for the reductions to amounts less than 100 ppm are reported in U.S. Pat. No. 4,079,118 entitled Method for Removing Nitrogen Oxides Using Ferricion-EDTA Complex Solutions issued Mar. 14, 1978, and various other wet processes have been developed to provide efficient removal of NO.sub.x. However, these processes generally require either the use of expensive starting materials or create a disposal problem for the products of the processes or both.
Numerous other patents have been issued which disclose wet processes for removal of NO.sub.x such as U.S. Pat. No. 3,984,522; U.S. Pat. No. 4,079,118 and U.S. Pat. No. 4,158,044. In addition, many patents have issued which disclose combined processes for removal of both SO.sub.2 and NO.sub.x. Examples of such patents include U.S. Pat. Nos. 4,126,529 and 4,347,227. Many other systems have been suggested, and the list is too long to include them all. However, there is much room for improvement in providing a practical, efficient removal process for both of such pollutants either individually or together.
As mentioned above, sulfur oxides can be effectively removed by flue gas desulfurization scrubbers. The majority of these scrubbers now in use involve wet limestone processes, which utilize aqueous slurries of limestone to neutralize the sulfurous and/or sulfuric acids produced from the dissolution and subsequent oxidation of flue gas SO.sub.2 in scrubbing liquors. The resulting solid slurries, containing CaSO.sub.3.1/2H.sub.2 O and gypsum (CaSO.sub.4.2H.sub.2 O), can be hauled away for disposal. Such practice is common among power plants located in areas where landfill space is abundant. On the other hand, the more practical solution for power plants situated in densely populated areas is to operate the scrubbers under forced oxidation conditions. Under those circumstances, the major by-product of the scrubbing process is gypsum, which is of some commercial value as a building material.
Further versatility in the processing by flue gas desulfurization scrubbers is obtained by utilizing other alkalis besides limestone or lime. These include soda ash (Na.sub.2 CO.sub.3), nahcolite (NaHCO.sub.3), trona (Na.sub.2 CO.sub.3 /NaHCO.sub.3), Na.sub.2 SO.sub.3, NaOH, KOH, K.sub.2 CO.sub.3 /KHCO.sub.3, magnesite (MgCO.sub.3), dolomite (CaCO.sub.3 /MgCO.sub.3), NH.sub.4 OH, and (NH.sub.4).sub.2 CO.sub.3 /NH.sub.4 HCO.sub.3. These materials are more expensive than limestone and are more often used in chemical industries where the volume of waste gas to be treated is small compared to those from power plants, and where the plants are in close proximity to the production sites of those alkalis.
While the wet flue gas desulfurization scrubbers described above are very efficient in the removal of SO.sub.2 from flue gas, they are incapable of removing sufficient NO because of its low solubility in aqueous solution. The installation of a separate scrubber for flue gas denitrification generally requires additional capital investment. Accordingly, approaches to modify existing wet flue gas desulfurization processes for the simultaneous removal of SO.sub.2 and NO.sub.x emissions have been under world wide investigation.
Several methods have been developed to enhance the absorption of NO.sub.x in scrubbing liquors. These include the oxidation of NO to the more soluble NO.sub.2 using oxidants such as O.sub.3, ClO.sub.2, and KMnO.sub.4, as well as the addition of various iron(II) chelates to the scrubbing liquors to bind and activate NO. So far, none of these methods has been demonstrated to be cost-effective, despite high removal efficiencies of both SO.sub.2 and NO.sub.x.